The present invention refers to a light concentrator, of the type used in particular in devices for the conversion of the radiant energy of the luminous radiation emitted by the sun into chemical, thermal or electrical energy, such as for example photovoltaic cells.
As is known, the use of solar energy presents technical and economic problems linked to the low energetic density of solar radiation, to its discontinuity (alternation of day/night, cycle of seasons, variation in the meteorological conditions) and to the modest value of the conversion yields (typically below 25%).
Such factors render the difference between the potential capacities and the practical possibilities of use significant. Among the various technologies perfected up to now for the exploitation of solar energy, photovoltaic technology is the most promising, in the medium or long term, by virtue of its characteristics of modularity, simplicity, reliability and reduced requirement for maintenance. The photovoltaic process, as is known, is based on the capacity of some suitably treated semiconductor materials, such as silicon, for generating electrical energy directly when they are exposed to solar radiation.
The conversion of solar radiation takes place with a yield of 12-15% in the photovoltaic cell; each cell is capable of producing around 1.5 watts at voltages of 0.6 volts; tens of electrically connected cells form a module (for 40-50 watts overall), which is the elementary component of the photovoltaic systems; more modules connected in series and/or in parallel are capable of providing the power required by the various applications.
One of the solutions currently used in the photovoltaic modules is that of concentration of the solar radiation: instead of using a photovoltaic cell of large dimensions, the concentration solution uses a concentrator of large dimensions which focuses the solar radiation onto a cell of reduced dimensions. This makes it possible, with parity of the area of the module exposed to radiation, to reduce the dimensions of the cell. In order for the concentration solution to be advantageous in economic terms, it is necessary that the saving on the cost of the photovoltaic cell is not entirely compensated by increments in the cost of the system. A concentration system in fact requires, in addition to the concentrator, a system for following, or “tracking” the solar disc, so that the module is always oriented in the correct direction; tracking the sun permits a doubling of the energy captured compared with concentrators in a fixed position. Tracking may be carried out both by means of analog control of electric motors arranged for movement of the module, and by means of digital control thereof, using a suitably programmed microcontroller. The same microcontroller may also detect the operating characteristics of the panel over time (temperature reached, electrical power produced or amount of water heated per unit of time) which it may then periodically transmit to a processor by means of a serial connection for displaying and storing the data.
Moreover, in the case of the concentration solution, the high density of incident radiation on the cell renders necessary a particularly efficient heat dissipation system in order to avoid loss of efficiency of the cell or even breakage of the latter.
The concentrator may work by reflection or by transmission. Transmission concentrators are typically formed of Fresnel lenses, inasmuch as a Fresnel lens guarantees the same capacity of concentration as conventional lenses but, with parity of diameter, has the advantage of having reduced thicknesses. This factor permits the construction of lenses by means of injection moulding processes without introducing deformations due to the removal of material deriving from the uneven thickness.
The Fresnel lenses conventionally used as concentrators in photovoltaic cells typically have a single focal length, i.e. focus all the incident radiation at one point of the optical axis placed a predetermined distance from the lens; this is obtained with flat Fresnel lenses, having a profile of the microreliefs of aspheric type, that is, such as to compensate for spherical aberration and to allow focusing of all the incident rays into a single point. In order to obtain uniform illuminance on the plane of the conversion cell it is typically sufficient for the focal plane of the lens and the plane in which the cell is positioned to be separated by a certain distance; this is equivalent to introducing a defocus, which produces a uniform distribution of irradiance on the plane of the cell.
This ideal situation, in which all the cell is uniformly illuminated, does not however take into account the polychromaticity of the incident radiation on the concentrator; as is known, the focal power of any optical element in transmission depends strongly on the wavelength, inasmuch as the refractive index of the material varies as the wavelength varies.
The wavelength at which the focal distance F of the lens is defined being called “reference wavelength” λ1, the focal distance at a general wavelength λ2 in paraxial approximation (i.e. with rays which spread at small angles with respect to the optical axis) varies according to the law:
      F    ⁡          (              λ        2            )        =            F      ⁡              (                  λ          1                )              ⁢                            n          ⁡                      (                          λ              1                        )                          -        1                              n          ⁡                      (                          λ              2                        )                          -        1            
In general, in regions of the spectrum that are characterised by a so-called “normal dispersion”, the refractive index decreases as the wavelength increases, and therefore the focal length increases. This sets practical limits on the ratio of concentration (ratio between the surface area of the lens and the surface area of the cell) that can be achieved with a concentrator in transmission; in fact, by way of example, if the concentrator is calculated to focus a green wavelength on the plane of the cell, a blue wavelength will be focused in a plane closer to the lens, while a red wavelength will be focused at a point further from the lens. The green will then be concentrated in a point region on the plane of the cell, while the red and the blue will have a broader distribution of irradiance; the dimension of the lens being defined, the dimensions of the cell cannot therefore be reduced indefinitely inasmuch as the chromatic dispersion would cause a large part of the red and blue radiation to fall outside the cell, compromising its efficiency. In order to cause more than 80% of the radiation to fall on the conversion cell, it is expedient for the value of the focal length to be very close to that of the concentrator-to-cell distance for an intermediate wavelength of the solar spectrum; the result is that the distribution of polychromatic irradiance becomes not very uniform and has a very high central peak; this peak persists even when varying the concentrator-to-cell distance (FIGS. 1a, b, c).
The presence of irradiance peaks on the plane of the cell is generally a problem, inasmuch as it means that the regions of the cell with higher irradiance reach very high temperatures, compromising both the conversion efficiency of the cell and the integrity of the lens itself.